Let \([.]\) denote the greatest integer function. If the domain of the function \( f(x) = \cos^{-1}\!\left(\frac{4x + 2[x]}{3}\right) \) is \([\alpha,\beta]\), then \( 12(\alpha + \beta) \) is equal to:
\( 6 \)
\( 8 \)
\( 9 \)
\( 4 \)